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Abstract:

The present invention relates to a method for performing a random access
procedure by a relay node operating in inband relaying mode in a wireless
or radio communication network, comprising at least one base station
adapted to communicate over a backhaul link with at least one relay node,
said relay node being adapted to communicate over an access link with at
least one user equipment, According to the present invention, the method
comprises the steps of: --Configuring resources for said back-haul link
and for said access link to be time-multiplexed at said relay node,
--Sending messages of said random access procedure on resources
configured for said backhaul link.

Claims:

1/ Method for performing a random access procedure by a relay node
operating in inband relaying mode in a wireless or radio communication
network, comprising at least one base station adapted to communicate over
a backhaul link with at least one relay node, said relay node being
adapted to communicate over an access link with at least one user
equipment, said method comprising: Configuring resources for said
backhaul link and for said access link to be time-multiplexed at said
relay node, Sending messages of said random access procedure on resources
configured for said backhaul link, and, Continuing communication on said
access link between said relay node and said user equipment while
performing said random access procedure on said backhaul link.

2/ (canceled)

3/ Method according to claim 1, wherein a random access response of said
random access procedure is sent from said base station to said relay node
in a MBSFN reserved sub-frame on said backhaul link, said relay node
configuring resources on said access link to be time multiplexed with
said MBSFN reserved sub-frame.

4/ Method according to claim 1, wherein a random access response of said
random access procedure is sent only to said relay node having started
said random access procedure on a resource configured in previous
operation by said base station for said backhaul link of said relay node.

5/ Method according to claim 1, wherein a random access response of said
random access procedure is sent to a group of relay nodes comprising said
relay node having started said random access procedure on a resource
configured in previous operation by said base station for said backhaul
link of said group of relay nodes.

6/ Method according to claim 1, further comprising the: allocating to
said relay node a random access channel resource comprising a random
access preamble univocally identifying said relay node; sending said
random access preamble identifying said relay node starting said random
access procedure to said base station on a random access channel; and
performing an identification of said relay node at said base station.

7/ Method according to claim 6, wherein a group of relay nodes are
identified by a random access preamble or a group of random access
preambles.

8/ Method according to claim 1, wherein said configuration of resources
for said backhaul link is performed by said base station.

9/ Method according to claim 1, wherein said configuration of resources
for said backhaul link is performed by an operation and maintenance
center.

10/ Relay node adapted to be used in a wireless or radio communication
network, said relay node being adapted to communicate over a backhaul
link towards at least one base station and over an access link towards at
least one user equipment, said relay node operating in inband relaying
mode, said relay node comprising: means for starting a random access
procedure while in connected mode communication on the access link; means
for receiving messages of said random access procedure on resources
configured for said backhaul link while continuing connected mode
communication on said access link.

11/ Relay node according to claim 10, wherein said means for starting a
random access procedure in connected mode on said access link comprises
means for sending a random access preamble in a random access channel on
a backhaul link while continuing connected mode communication on said
access link.

12/ Relay node according to claim 10, wherein said relay node has control
of the access link for said user equipment connected to the relay node.

13/ Relay node according to claim 10, further comprising means to store
data received from said user equipment over the access link while said
relay node is performing the random access procedure and means for
sending said stored data upon completion of said random access procedure
over said backhaul link.

14/ (canceled)

Description:

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the field of telecommunication,
and more specifically to a method for performing a random access
procedure by a relay node in a wireless or radio communication network.

[0002] Relay-based wireless or radio communication network are gaining in
interest thanks to the possibility of reaching enhanced coverage while
keeping the energy required in addition in range. More precisely,
coverage of high data rates, group mobility, temporary network deployment
cell edge throughput and/or coverage in new areas are subjects which can
be greatly improved by introducing relays in conventional wireless/radio
communication networks.

[0003] Such relays based wireless or radio communication networks are
especially being investigated in the framework of 3GPP LTE (Long Term
Evolution) or IEEE 802.16j/m standardization initiatives.

[0004]FIG. 1 is showing the general architecture of relay-based
wireless/radio communication networks where a user equipment UE no more
communicates directly with a base station eNB but where a relay node RN
is included in the path between user equipment UE and base station eNB.
Consequently, relay node RN is wirelessly connected to base station eNB
also called donor cell or donor eNB over a backhaul link also called Un
interface in the context of 3GPP LTE standardization and relay node is
wirelessly connected to user equipment UE over an access link also called
Uu interface in the context of 3GPP LTE standardization.

[0005] There are different types of relay operation on the backhaul
interface. On the one hand, inband operation consists in that the
backhaul link shares the same band as the link used for direct
communication between the user equipment UE and the base station eNB. On
the other hand, outband operation consists in that the backhaul link
operates in a different band that the one used for direct link between
the user equipment and the base station.

[0006] In the context of the present invention, we will consider only the
inband relay operation also called type 1 relaying. More precisely, a
type 1 relay node controls a cell which appears to the user equipment as
a cell distinct from the donor cell. The relay node has consequently its
own physical cell id and transmits its own synchronization channel and
reference symbols, as such it appears as a base station eNB to the user
equipments.

[0007] Due to the fact that for in band relaying, the backhaul link and
the access link operate in the same frequency band, the relay transmitter
would cause interference on its relay receiver if they are active
simultaneously and/or if not sufficient isolation of the incoming and
outgoing signals are provided. This self interference issue is especially
problematic for random access procedures performed by the relay node.

[0008] Except for the initial access of the relay node to the network from
the RRC-IDLE state after the initial power up, following situations may
trigger a Random Access (RA) procedure at the relay node: [0009] 1. DL or
UL data arrival during RRC_CONNECTED when UL is unsynchronised. [0010] 2.
UL data arrival while no dedicated PUCCH resources available for D-SR
transmission [0011] 3. Handover [0012] 4. The maximum number of
transmission is reached for the D-SR transmission [0013] 5. RRC
Connection re-establishment procedure

[0014] In general, the Random Access Procedure consists in sending in an
uplink channel (Random Access Channel RACH) control information from a
user equipment/or relay node to the network. The RACH channel is a
contention based channel where several user equipment/relays might access
the same resource. The network, then, answers with a random access
response which is comprised within a flexible time window configured by
the base station and contains at least uplink grant on resources
available between the user equipment and the base station so that the
user equipment can schedule transmission on the granted uplink resources.

[0015] The random access procedure in situations 1 to 5 occurs when the
relay node is already operating on the access link. Unfortunately, the
usual random access procedure on the backhaul link will interfere with
the operation on the access link so that it is necessary for the relay
node to stop all communication on the access link interface while
performing the random access procedure on the backhaul link.

[0016] An illustration of the flow diagram of the currently possible
random access procedure by the relay node while in service to user
equipment UE already attached to the relay node is illustrated on FIG. 2.

[0017] A step 1 consists in detecting at the relay node RN one of the
situations 1) to 5) in which a random access procedure should be
triggered

[0018] Step 2 consists in stopping all uplink and downlink communication
on the access link with user equipments

[0020] Step 4 consists in receiving from the base station a random access
response which is comprised within a flexible time window configured by
the base station and contains at least uplink grant on resources
available between the user equipment and the base station so that the
user equipment can schedule transmission on the granted uplink resources

[0022] Step 6 consists in a resource configuration message sent to the
relay node indicating a new resource configuration valid for the
communication on the backhaul link from this time on.

[0023] Even if the time of the random access preamble transmission in step
3 is well defined by the random access procedure (PRACH configuration),
the relay node should expect the reception of messages 4 and 6 at any
time not further precisely defined by the usual random access procedure.
Also the relay node should expect to transmit the message 3 at any time
on the granted uplink resources. That is why to avoid interference with
message 4 and 6 on the access link it is necessary to stop the
communication on the access link.

[0024] This presents the disadvantage to degrade the end user experience
for user equipment connected to the network over the relay node. The user
equipment will experience sudden and long interruptions of the service.
The duration of the interruption will be equal to the time necessary to
configure the stopping of the access link, the time to configure the
resuming of the access link and the time necessary to the perform the
random access procedure.

[0025] A particular object of the present invention is to provide for a
solution to reduce the interruption time on the access link while the
relay node is to perform a random access procedure on the backhaul link.

[0026] Another object of the invention is to provide a relay node and a
base station adapted to support such mechanisms.

SUMMARY OF THE INVENTION

[0027] The present invention is directed to addressing the effects of one
or more of the problems set forth above. The following presents a
simplified summary of the invention in order to provide a basic
understanding of some aspects of the invention. This summary is not an
exhaustive overview of the invention. It is not intended to identify key
or critical elements of the invention or to delineate the scope of the
invention. Its sole purpose is to present some concepts in a simplified
form as a prelude to the more detailed description that is discussed
later.

[0028] These objects presented above, and others that appear below, are
achieved in particular by a method for performing a random access
procedure by a relay node according to claim 1, a relay node according to
claim 10 and a base station according to claim 14.

[0029] According to the present invention, a configuration of the
resources for the backhaul link and for the access link is performed
before the random access procedure starts so that both are time
multiplexed. This configuration of resources makes sure that there is no
interference between the access and the backhaul link. Then messages of
the random access procedure are sent on resources configured for the
backhaul link between the base station and the relay node.

[0030] According to the invention, to handle the interference problem the
relay node is not communicating with user equipments when it is supposed
to receive a random access response from the donor cell.

[0031] In a first embodiment, this is realized by creating gaps in the
access link during which user equipments are not supposed to expect any
relay transmission. These gaps can be created by configuring MBSFN
sub-frames for sole use by the relay nodes which the user equipments are
not expected to follow.

[0032] In a second embodiment, the backhaul link respectively access link
configuration which was used before the random access procedure by the
relay node continue to be used so that the relay nodes receives the
random access procedure messages on the backhaul interface not
simultaneously with the operation on the access link.

[0033] Either the random access response is sent to the sole relay node
which has entered the random access procedure or the random access
response is sent to a group of relay nodes all having started the random
access procedure.

[0034] The method according to the present invention presents the
advantage to ensure that the relay node can continue to communicate on
the access link without experiencing self interference from the random
access procedure until a new configuration of the backhaul link/access
link resources is reached at the end of the random access procedure.

[0035] Another advantage of the present invention consists in that the
relay node stores any data received on the access link from the user
equipments and can resume the data transmission/reception over the
backhaul link as soon as the random access procedure according to the
present invention has been successfully completed. Consequently, the
interruption seen by the user equipment connected to the relay node is
negligible.

[0036] Further advantageous features of the invention are defined in the
dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other characteristics and advantages of the invention will appear
on reading the following description of a preferred embodiment given by
way of non-limiting illustrations, and from the accompanying drawings, in
which:

[0038]FIG. 1 shows a simplified architecture of a relay-based wireless
radio communication network;

[0040]FIG. 3 shows a random access procedure according to a first
embodiment of the present invention;

[0041]FIG. 4 shows the structure of a MBSFN subframe as used in
combination with the first embodiment of the present invention.

[0042]FIG. 5 shows a random access procedure according to a second
embodiment of the present invention;

[0043]FIG. 6 shows a random access procedure according to a third
embodiment of the present invention;

[0044] FIG. 7 shows an embodiment of a relay node according to the present
invention;

[0045]FIG. 8 shows an embodiment of a base station according to the
present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0046] The functions of the various elements shown in the Figures,
including any functional blocks labeled as `processors`, may be provided
through the use of dedicated hardware as well as hardware capable of
executing software in association with appropriate software. When
provided by a processor, the functions may be provided by a single
dedicated processor, by a single shared processor, or by a plurality of
individual processors, some of which may be shared. Moreover, explicit
use of the term `processor` or `controller` should not be construed to
refer exclusively to hardware capable of executing software, and may
implicitly include, without limitation, digital signal processor (DSP)
hardware, network processor, application specific integrated circuit
(ASIC), field programmable gate array (FPGA), read only memory (ROM) for
storing software, random access memory (RAM), and non volatile storage.
Other hardware, conventional and/or custom, may also be included.
Similarly, any boxes shown in the Figures are conceptual only. Their
function may be carried out through the operation of program logic,
through dedicated logic, through the interaction of program control and
dedicated logic, or even manually, the particular technique being
selectable by the implementer as more specifically understood from the
context.

[0047]FIG. 1 and FIG. 2 have been described in connection with prior art.

[0048]FIG. 3 shows a random access procedure according to a first
embodiment of the present invention.

[0049] Step 31 consists in triggering the random access procedure by the
relay node. This triggering is preferably done by sending a random access
preamble which is univocally assigned to the relay node on a random
access channel at a predefined sub frame of the random access channel
defined by PRACH configuration as defined in release 8 random access
procedure of 3GPP LTE.

[0050] Alternatively, step 31 consists in triggering the random access
procedure by the relay node. This triggering is preferably done by
sending a random access preamble which is assigned to a group of relay
nodes that the relay node belongs to on a random access channel at a
predefined sub frame of the random access channel defined by PRACH
configuration as defined in release 8 random access procedure of 3GPP
LTE.

[0051] In a further alternative, step 31 consists in triggering the random
access procedure by the relay node. This triggering is preferably done by
sending a random access preamble out of a group of random access
preambles assigned to a group of relay nodes that the relay node belongs
to on a random access channel at a predefined sub frame of the random
access channel defined by PRACH configuration as defined in release 8
random access procedure of 3GPP LTE.

[0052] Since the relay node has the control over the access link, the
relay node can ensure that no data are sent or received over the access
link while it intends to send its random access preamble over the
backhaul link.

[0053] Alternatively, to send the random access preamble, it may be
envisaged that the relay node uses a previously configured resource
reserved for communication over the backhaul link. The configured
resource may be controlled by the base station or alternatively may be
determined centrally at an operation and maintenance center and then
preferably communicated to both relay node and base station.

[0054] This presents the advantage that the relay node entering the random
access procedure does not need to perform any change on the configuration
of the access link which was previously performed. The previous resource
configuration is already foreseen to avoid any collision between the
access link communication and the backhaul link communication.

[0055] In other contexts, it will be clear for a person skilled in the art
that other types of relay node identifiers different from the random
access preamble can be exchanged on other types of channel between the
relay node and the base station in order to trigger the random access
process.

[0056] Upon reception of the trigger message, the base station identifies
the relay node which has started the random access procedure preferably
by mapping the received the random access preamble with the corresponding
relay node.

[0057] Upon reception of the trigger message, the base station identifies
the group which the relay node belongs to where the relay node has
started the random access procedure preferably by mapping the received
the random access preamble with the corresponding group of relay nodes.

[0058] Step 32 consists in sending a random access response from the base
station to the relay node.

[0059] According to this embodiment of the present invention, a MBSFN sub
frame is used to convey the random access response. The MBSFN sub frame
is defined so as to contain broadcast multicast information. Some sub
frames are reserved in the context of Release 8 3GPP LTE to contain only
MBSFN and are consequently only relevant on the backhaul link. These
categories of sub frames are not destined to the user equipments. Only
the first 2 control symbols of the MBSFN sub frame may be decoded by the
user equipments. Consequently, the relay node in control of the
configuration of the access link will not schedule any communication on
the access link while MBSFN frames are exchanged on the backhaul link.
The MBSFN sub-frames are a good example of sub frames fulfilling the
condition that no communication takes place on the access link and on the
backhaul link simultaneous. This embodiment makes use of already existing
sub frames for the sake of an efficient random access procedure.

[0060] More precisely, the random access response may be sent on the
downlink synchronisation channel (DL-SCH) and transmitted on MBSFN sub
frames reserved for relay node(s) operation. This message could be
semi-synchronous with the triggering message of step 31. The random
access response could be addressed to RA-RNTI (Random Access Radio
Network Temporary Identity) on the physical downlink control channel
(PDCCH). RA-RNTI is corresponding to the PRACH where the preamble is
transmitted; hence it can be received by a group of UE/RN who have
transmitted preambles on the corresponding PRACH. A group of relay nodes
who have transmitted preambles on the corresponding PRACH can receive the
random access response transmitted on MBSFN subframes. Therefore, this
method can be used to send the random access response message to a group
of RNs. The random access response message would convey at least:
[0061] timing alignment information [0062] random access-preamble
identifier [0063] UL Grant on backhaul link UL resources of the relay
node [0064] intended for one or multiple relay nodes in one DL-SCH
message

[0065] Step 33 consists in sending a first scheduled uplink transmission
between the relay node and the base station on an uplink resource
indicated in the previously received random access response message in
step 32. In more details, the first scheduled UL transmission on UL-SCH
preferably uses HARQ, the size of the transport blocks depends on the UL
grant conveyed in step 32.

[0070] For other events it includes an uplink Buffer Status Report when
possible and/or cell (specific) radio network temporary identity (C-RNTI)
of the relay node.

[0071]FIG. 4 shows the structure of a MBSFN sub-frame as used in
combination with the first embodiment of the present invention. More
precisely, it shows a succession of 2 sub-frames 41 and 42. Each
sub-frame comprises a control part Ctrl and a data part 411, 421.
Sub-frame 41 is a usual sub-frame containing usual user data in data part
411 while sub-frame 42 is a MBSFN sub-frame containing broadcast data in
data part 421. According to the present invention, the data part 421 of
MBSFN sub-frame 42 is used to convey the random access response message
from the base station to the relay node during the random access
procedure. The characteristic of the data part of MBSFN sub-frame is that
it is not destined to be accessed by any user equipment is it
transporting. Such MBSFN sub-frames are preferably reserved in a frame.

[0072]FIG. 5 shows a random access procedure according to a second
embodiment of the present invention.

[0073] This random access procedure comprises steps 51, 52, and 53. Step
51 and 53 are identical to steps 31 and 33 already described in relation
with FIG. 3. The difference consists in step 52 where the random access
response message is addressed to the relay node cell identity (eg C-RNTI
Cell (specific) Radio Network Temporary Identity) and sent on the R-PDCCH
of the relay node in a resource configured previously to the entry in the
random access procedure as reserved for communication on the backhaul
link for the relay node. The configured resource may have been controlled
by the base station or alternatively may have been determined centrally
at an operation and maintenance center and then preferably communicated
to both relay node and base station prior to the entry in the random
access procedure.

[0074] All other aspects of the random access response as for example the
information contained are similar to the one already presented in
connection with step 32.

[0075] In this embodiment, the random access response is sent individually
to each individual relay node which has entered the random access
procedure separately.

[0076]FIG. 6 shows a random access procedure according to a third
embodiment of the present invention.

[0077] This random access procedure comprises steps 61, 62, and 63. Step
61 and 63 are identical to steps 51 and 53 already described in relation
with FIG. 5. The difference between the second and this third embodiment
consists in addressing the random access response message to the relay
node group identity (eg R-RA-RNTI (Relay Random Access Radio Network
Temporary Identity) random access RN-RNTI (Relay Node Radio Network
Temporary Identity) for relay nodes) and sending it on the R-PDCCH in a
resource configured previously to the entry in the random access
procedure as reserved for communication on the backhaul link for a group
of relay nodes. The configured resource may have been controlled by the
base station or alternatively may have been determined centrally at an
operation and maintenance center and then preferably communicated to both
relay node and base station.

[0078] In this embodiment, the random access response is sent to a group
of relay nodes having entered the random access procedure.

[0079] This presents the advantage of generating one single message for a
whole group of relay nodes all having started the random access
procedure. This saves resources used for the random access procedure.

[0080] FIG. 7 shows an embodiment of a relay node according to the present
invention. The relay node comprises an interface 71b towards the backhaul
link to communicate with base stations, and an interface 71a towards the
access link to communicate with user equipments. The relay node comprises
a module 72 for detecting the need for triggering a random access while
in connected mode on the access link. Events monitored by module 72 are
for example: [0081] 1. DL or UL data arrival during RRC_CONNECTED when UL
is unsynchronised. [0082] 2. UL data arrival while no dedicated PUCCH
resources available for D-SR transmission [0083] 3. Handover [0084] 4.
The maximum number of transmission is reached for the D-SR transmission
[0085] 5. RRC Connection re-establishment procedure

[0086] Module 72 is further adapted to generate a random access trigger
message to be sent on the backhaul link. This trigger message is
preferably containing a dedicated random access preamble and sent on the
RACH or on a dedicated resource previously allocated to the relay node
for communication on the backhaul link.

[0087] Module 72 is further adapted to generate a random access trigger
message to be sent on the backhaul link. This trigger message is
preferably containing a random access preamble out of a pre-assigned
group of random access preambles and sent on the RACH or on a dedicated
resource previously allocated to the relay node for communication on the
backhaul link.

[0088] Module 72 is connected to module 74 adapted to control the access
interface. Module 74 is only impacted by the present invention when
module 72 uses the RACH for transmitting the trigger message. In this
case module 74 should configure the access link so as not to get any
activity on the access link while transmitting the random access trigger
message on the backhaul interface.

[0089] Relay node further comprises a module 73 for receiving message of
the random access procedure on resources configured for the backhaul link
before the random access procedure has started. The configured resource
may have been controlled by the base station or alternatively may have
been determined centrally at an operation and maintenance center and then
preferably communicated to the relay node.

[0090] Especially, the random access response of the random access
procedure is received by module 73 either as part of a MBSFN sub frame or
as a message addressed to the relay node or a group of relay nodes as
previously described in connection with the detailed description of 3
embodiments of the method for performing the random access procedure at a
relay node.

[0091] Relay node further comprises a data storage module 75 for storing
data received on the active access link from user equipment and keep them
for the duration of the random access procedure on the backhaul link.
Data storage module is adapted to forward the stored data upon completion
of the random access procedure on the backhaul link.

[0092]FIG. 8 shows an embodiment of a base station according to the
present invention. The base station comprises a module 81 for configuring
resources for the backhaul link between the base station and the relay
node. This configuration of resources is primarily used when the relay
node is in connected mode on the backhaul link and on its access link.
Further the base station comprises a module 82 for receiving an
indication that a relay node is starting a random access procedure while
it is remaining in connected mode on its access link. This indication may
be a message containing a dedicated random access preamble.
Alternatively, this indication may be a trigger message is preferably
containing a random access preamble out of a pre-assigned group of random
access preambles and sent on the RACH or on a dedicated resource
previously allocated to the relay node for communication on the backhaul
link.

[0093] Then, the base station comprises a module 83 for sending messages
of said random access procedure on resources configured by module 81 for
the backhaul link of the relay node. Doing this, the base station ensures
that the random access procedure messages will not be colliding with the
messages exchanged on the access link of the relay node. Module 83 sends
preferably a random access response either as part of the MBSFN sub frame
or on a relay node specific allocated resource on the backhaul link
specifically addressed to the relay node or addressed to a group of relay
nodes as already described in combination with the method for performing
random access procedure at a relay node.